Arboricultural Consultancy

Subsidence

Root barrier solutions for subsidence

Root barriers are used in the prevention of roots from nearby trees and vegetation causing damage to new and existing buildings and infrastructure. DENDRO-SCOTT™ Root Barrier is available for in standard sizes for small to medium scale projects and also, as in the photos below, in custom sizes for large developments. Read on to find out more about the causes of subsidence and why root barrier makes an excellent solution.

What is subsidence?

Subsidence is the downward movement of the ground supporting the building. Particular problems arise when the movement varies from one part of the building to another.

The causes of subsidence

Clay soils
Clay soils are particularly vulnerable to subsidence, as they shrink and swell depending on their moisture content

Vegetation
Trees and shrubs take moisture from soils, causing them to shrink. This is especially true during long periods of dry weather, as roots extend in search of water

Less commonly, problems may occur where properties are built over, or close to, mine workings

Is my property suffering from subsidence?

It is the sight every homeowner dreads: cracks appearing in the walls of their home. But do not fear the worst! While there has been an increase in subsidence claims in recent years, most buildings suffer minor cracking at some time, so it does not necessary mean that there is a subsidence problem.

As soon as you believe there may be a problem, you should contact your buildings insurer. A policy will normally require the insurer to be advised of any potential claim as soon as possible and, in any event, it is sensible because the sooner the problem is investigated, the quicker everything can be put right and the less inconvenience will be caused.

It will first be necessary to identify the cause of the damage and what needs to be done to stop it. Stabilisation of the situation will need to be carried out before any remedial action can be carried out. Only after any movement has been stabilised can the necessary repairs can be carried out.

Insurers really do understand and appreciate policyholders’ concerns and will do all they can to minimise the worry and inconvenience. Insurers will also keep policyholders informed of developments at every stage.

In the UK the shrinkage and swelling of clay soils, particularly when influenced by trees, is the single most common cause of foundation movements which damage domestic buildings. Currently, the value of insurance claims for subsidence and heave damage to domestic properties amounts to almost £400 million per annum. While the problem primarily affects the older existing building stock, it is also a potential problem for new construction with implications for design, detailing and construction quality control to prevent it becoming a real problem.

The problem with Clay Soils

Clay Soils change their volume in relation to their water content. They can shrink as they become drier and expand as they become wetter. They act like a sponge. Some types of clay shrink and expand more than other types. It is possible to determine the exact shrinkability of a clay soil by taking a sample and having it analysed in a laboratory. The laboratory will carry out a number of tests and calculate the soils plasticity index (PI). The plasticity index relates proportionally to the shrinkage potential of the soil.

A clay soil will shrink and expand in reply to climatic changes associated with vegetation. In summer, where rainfall is reduced and evapotranspiration is higher, a clay soil will shrink. In winter, rainfall increases and evapotranspiration decreases, the soil increases its volume. This is a natural process called Seasonal Fluctuations. Factors such as the type and species of vegetation, drainage and wind can modify the level of these fluctuations.

The effect of types of vegetation and distances to property

As trees cause clay soils to shrink by drawing water through their roots, predominantly during spring and summer, this shrinkage results in both vertical and horizontal ground movements that, if transmitted to a building’s foundations, can cause damage to the building structure. The amount of shrinkage depends on the type of clay soil, the type and size of tree, and on climate. Long dry periods result in the greatest movements particularly when, as was the case in 1989/90, two dry summers are separated by a relatively dry winter. Trees growing under grass cover are forced to compete for their water and to extract water from greater depths than they might otherwise do.

A clay soil which has lost additional water due to vegetation and is below its field capacity is termed as being desiccated and having a Soil Moisture Deficit.

A clay soil which fails to return to its field capacity after the wet season and enters the next dry season with a soil moisture deficit already in place, is termed as having a Persistent Moisture Deficit sometimes known as a Permanent Water Deficit.

A persistent moisture deficit may develop as the tree grows. That is, the ground is unable to rehydrate completely over the winter months. The establishment of a persistent moisture deficit results in shrinkage of the soil and consequent subsidence of the ground. Where a persistent deficit has been established, tree removal will result in rehydration of the ground. Water contents at depth will return to values close to their original ones, accompanied by soil swelling and ground heave.

Avoid tree felling with a root barrier

There’s good news. Installation of a root-barrier will allow the ground to return to its field capacity without underpinning and without felling the tree. Vertical root barrier solutions for subsidence prevent the roots from draining the soil close to buildings and at greater depths beneath buildings. DENDRO-SCOTT™ Root Barrier can be used in vertical or horizontal installations. Learn more here about DENDRO-SCOTT™ Root Barrier Solutions.